The present invention relates to optical amplification apparatus for use in optical transmission systems and so on.
As a reduction in cost has been demanded for optical transmission systems, a wavelength-multiplex optical transmission has been taken into consideration for transmitting signal light at one or more mutually different wavelengths on a single transmission fiber. It is also thought that an amplifier suitable for use in such wavelength-multiplex optical transmission is an optical amplification apparatus which has a wide amplification wave band and is capable of achieving the amplification with less noise.
It is known, however, that a rare metal added optical fiber and a semiconductor optical amplifier constituting the above-mentioned optical amplification apparatus have a gain dependency so that optical outputs and gains at respective wavelengths present deviations due to the difference in wavelengths after amplification. For this reason, the optical power at different wavelengths after transmission involves a deviation due to the difference in wavelengths. Particularly, if a number of optical amplifiers are used to relay signal light at multiple stages, the deviation of optical power between different wavelengths, generated at respective relay stages, are accumulated as the signal light is relayed from one stage to next, thus increasing the deviation of optical power between the different wavelengths.
In the wavelength-multiplex optical transmission, since the wavelength signal having the lowest power of all multiplexed wavelengths must be regarded as a lower limit value of received power after transmission, a maximum transmission distance in the wavelength-multiplex transmission is limited by the wavelength signal having the lowest power. Thus, it is of great importance to reduce the deviation of power between different wavelengths in the output of an optical amplification apparatus, in order to extend a maximum relay transmission distance.
To solve this problem, an article titled "Collective Smoothing of Multiple Wavelength Amplification Characteristics of Fiber Optic Amplifier Using Fiber Amplification Ratio Control" Technical Reports of the Institute of Electronics, Information and Communications OCS94-66, OPE94-88 (1944-11) has proposed the following technique.
FIG. 1 illustrates the configuration of an optical amplification apparatus according to the technique disclosed in the article. Referring specifically to FIG. 1, the optical amplification apparatus includes an erbium-added optical fiber 51, an optical isolator 52, a light combiner 53, an excitation light source 54, an optical attenuator 55, an optical coupler 56 for splitting the output of the optical attenuator 55, and a light detector 57 for detecting split light.
In the disclosed technique, the illustrated optical amplification apparatus is controlled by an auto fiber gain controller (AFGC) such that a fiber gain remains at 12 dB, thereby minimizing a deviation of gain between respective wavelengths. In addition, an auto power controller (APC) implemented by the optical attenuator 55 is used to prevent a change in relay amplification ratio from affecting the gain spectrum.
It has been reported that, according to theoretical calculations, the optical amplification apparatus presented a minimum gain deviation between respective wavelengths, which is 0.12 dB or less, when the erbium-added optical fiber 50 had a length of 11 meters, assuming that a deviation of gain between the respective wavelengths of input light was 0 dB. It has been also reported that after the optical amplification apparatus has been used to relay light having four different wavelengths multiplexed thereon 60 times, a gain deviation was 1.5 dB or less.
Optical losses during transmission may vary from one case to another due to a difference in fiber loss within each relayed area, a difference in optical power between adjacent wavelengths, and so on. Additionally, in an actual use, relayed distances and fiber losses in respective areas are not always constant. It is therefore difficult to predict a deviation of gain between respective wavelengths and optical power at the respective wavelengths in an actual use. Therefore, the optical amplification apparatus illustrated in FIG. 1 has a problem in an actual use that if an input level changes or if a deviation of gain occurs between input wavelengths, the optical amplification apparatus cannot reduce a deviation of gain between output wavelengths to 0 dB.
Also, when the optical amplification apparatus illustrated in FIG. 1 is used, if an independent fluctuation in output power of signal light at a certain wavelength caused by an external factor, for example, is to be suppressed, stable output power of signal light at the remaining wavelengths is also suppressed simultaneously, thus adversely affecting the stability of the output power of the signal light at the different wavelengths.
Further, since the optical amplification apparatus illustrated in FIG. 1 is dependent on the gain thereof for establishing an optimal condition for eliminating the deviation of gain between wavelengths, it cannot freely set outputs of signal lights. More specifically, since a relayed distance is limited by the optical amplification apparatus the freedom in designing the architecture of a transmission system is restricted. The optical amplification apparatus illustrated in FIG. 1 additionally has a problem that it must be optimized to eliminate a deviation of gain between wavelengths in each relay area.